Petrochemicals: steam cracking for ethylene and propylene

Steam cracking for ethylene and propylene: free-radical chemistry, furnace severity, dilution steam, TLE quench, the recovery train, and conversion-selectivity-recycle balances.

Drafted with Aria, reviewed by the AiCanCode.org team. Spotted an error? Use Give Feedback at the bottom of the page.

Why it matters

Ethylene and propylene are the two largest-volume organic chemicals and the starting point for polyethylene, polypropylene, PVC, ethylene glycol, styrene, acrylonitrile and much more. Steam cracking makes almost all ethylene and much of the world's propylene, and a cracker is one of the most energy-intensive units in the chemical industry, so furnace severity, feed choice and the separation train decide profitability.

Key ideas

Chemistry. Steam cracking is non-catalytic thermal cracking by free-radical chain reactions (initiation by C–C bond breaking, propagation by H-abstraction and β-scission, termination by radical combination). The primary reactions are strongly endothermic and increase the number of moles, so they are favoured by high temperature and low hydrocarbon partial pressure. Secondary reactions (condensation to aromatics, heavy oils and coke) are favoured by long residence time and high partial pressure, so the reaction is stopped quickly.

  • Ethane: C₂H₆ → C₂H₄ + H₂ (main), with some CH₄, C₃ and heavier.
  • Naphtha gives a broad slate: H₂, CH₄, C₂H₄, C₃H₆, C₄ (butadiene), pyrolysis gasoline (rich in benzene) and fuel oil.

Feedstocks. Ethane and propane (from natural gas) give the highest ethylene yield (about 80 % ultimate yield from ethane with recycle) and few co-products. Naphtha (common in India) gives about 28–32 % ethylene and 14–17 % propylene by mass plus valuable C₄ and aromatics. Gas oils give lower ethylene and more fuel oil and coke.

Furnace (cracking heater).

  • Feed is preheated in the convection section, mixed with dilution steam and heated further, then cracked in radiant coils. Coil outlet temperature is about 800–870 °C, residence time about 0.1–0.5 s, coil outlet pressure about 1.7–2.5 bar absolute.
  • Dilution steam (about 0.3 kg/kg for ethane, 0.5 kg/kg for naphtha, higher for gas oil) lowers hydrocarbon partial pressure, improving olefin selectivity, and reduces coking by reacting with coke and passivating the metal surface. It is not the heat source; heat comes from burners through the coil wall.
  • Severity: higher temperature or longer residence time raises conversion and ethylene but lowers propylene. The propylene-to-ethylene ratio (or methane yield) is used as a severity index.
  • Transfer line exchangers (TLEs) quench the gas within milliseconds to below about 400–600 °C to freeze the product distribution, raising high-pressure steam.
  • Coke builds up on coil walls, raising tube metal temperature and pressure drop, so coils are decoked every few weeks with steam and air.

Recovery section.

  1. Oil quench and primary fractionator (liquid feeds) removing fuel oil; water quench tower condensing dilution steam and pyrolysis gasoline.
  2. Cracked-gas compression in 4–5 stages to about 35 bar, with inter-stage cooling.
  3. Caustic or amine wash to remove H₂S and CO₂; drying on molecular sieves.
  4. Chilling train and demethaniser (cryogenic, about −100 °C) to separate H₂ and CH₄.
  5. Deethaniser, acetylene hydrogenation (Pd catalyst) and C₂ splitter (very tall column, about 100+ trays, because ethylene and ethane have close volatility) giving polymer-grade ethylene; ethane is recycled to the furnaces.
  6. Depropaniser, methylacetylene/propadiene hydrogenation and C₃ splitter for polymer-grade propylene; then debutaniser for the C₄ cut (butadiene extraction) and pyrolysis gasoline (to BTX recovery).

Other propylene routes: FCC off-gas, propane dehydrogenation and metathesis supplement steam cracking.

Formulas

C₂H₆ → C₂H₄ + H₂ (ΔH ≈ +137 kJ/mol; endothermic)

Conversion X = (feed in − feed out) / feed in Selectivity S = mol product formed / mol feed converted (molar; specify mass or mole basis) Yield Y = X × S (per pass)

Mass of ethylene = (m_feed / M_feed) × X × S × M_ethylene

  • m: mass (kg), M: molar mass (C₂H₆ 30.07, C₂H₄ 28.05 g/mol).

Recycle with complete recovery of unconverted feed: fresh feed = product / S_molar (overall conversion = 100 %), furnace feed = fresh / X.

Dilution steam = steam ratio (kg/kg) × hydrocarbon to furnace (kg)

Worked examples

Example 1 (standard): per-pass ethylene yield. Ethane is cracked at 1000 kg/h with 65 % conversion and 80 % molar selectivity to ethylene. Find the ethylene produced.

  1. Ethane moles = 1000 / 30.07 = 33.26 kmol/h.
  2. Converted = 0.65 × 33.26 = 21.62 kmol/h.
  3. Ethylene = 0.80 × 21.62 = 17.29 kmol/h × 28.05 = 485 kg/h.

Answer: about 485 kg/h of ethylene (48.5 % mass yield per pass).

Example 2 (GATE level): ethane cracker with recycle. A plant must make 100 t/day of ethylene. Per-pass ethane conversion is 60 %, molar selectivity to ethylene 82 %, and unconverted ethane is fully recycled. Dilution steam is 0.3 kg per kg of ethane to the furnaces. Find fresh ethane, furnace feed, recycle and dilution steam.

  1. Ethylene = 100 000 / 28.05 = 3565 kmol/day.
  2. With full recycle all fresh ethane is eventually converted: fresh = 3565 / 0.82 = 4348 kmol/day = 4348 × 30.07 = 130.7 t/day.
  3. Furnace feed = fresh / X = 4348 / 0.60 = 7246 kmol/day = 217.9 t/day.
  4. Recycle = 217.9 − 130.7 = 87.2 t/day.
  5. Dilution steam = 0.3 × 217.9 = 65.4 t/day.

Answer: fresh ethane 131 t/day, furnace feed 218 t/day, recycle 87 t/day, steam 65 t/day.

Common mistakes

  • Saying steam supplies the heat of cracking or acts as a catalyst; it is a diluent and coke suppressant.
  • Thinking high pressure helps; the reactions increase moles, so low partial pressure is wanted.
  • Mixing up mass yield and molar selectivity; always state the basis.
  • Using per-pass conversion as overall conversion in a recycle plant.
  • Forgetting the TLE quench; slow cooling lets secondary reactions destroy olefins.
  • Placing the C₂ splitter before acetylene removal; acetylene must be hydrogenated to meet polymer-grade specs.

For GATE CH

Expect questions on why high temperature, short residence time and dilution steam are used, feedstock effects on yields, severity, the role of the TLE, the sequence of columns in the recovery section, and impurities removed (acetylene, CO₂, H₂S). Numericals cover conversion–selectivity–yield, recycle balances and steam ratios.

Quick check

  1. What reaction mechanism operates in steam cracking?
  2. Why is dilution steam added?
  3. Which feed gives the highest ethylene yield?
  4. Why is the C₂ splitter so tall?

Answers: 1. free-radical chain reactions; 2. to lower hydrocarbon partial pressure (better olefin selectivity) and reduce coking; 3. ethane; 4. ethylene and ethane have a relative volatility close to 1, so many stages are needed.

Interview questions

All Chemical Technology interview questions

Try answering each one aloud before you open it.

  1. 1.What is steam cracking in the context of petrochemicals?Concept

    Steam cracking is a petrochemical process used to break down large hydrocarbon molecules into smaller ones, primarily to produce ethylene and propylene. It involves heating hydrocarbons, such as naphtha or ethane, in the presence of steam at high temperatures, typically between 750°C and 900°C. This process results in the breaking of carbon-carbon bonds, leading to the formation of smaller alkenes.

  2. 2.Explain the importance of ethylene and propylene in the petrochemical industry.Concept

    Ethylene and propylene are crucial building blocks in the petrochemical industry. Ethylene is used to produce polyethylene, which is the most common plastic, as well as ethylene oxide and ethylene glycol, which are used in antifreeze and polyester production. Propylene is used to produce polypropylene, a versatile plastic, and other chemicals like acrylonitrile and propylene oxide. Their wide range of applications makes them essential for manufacturing various consumer and industrial products.

  3. 3.Why is steam used in the steam cracking process?Application

    Dilution steam lowers the hydrocarbon partial pressure; because cracking increases the number of moles, this raises olefin selectivity and suppresses the bimolecular secondary reactions that make heavy aromatics and coke. Steam also gasifies some coke (C + H2O -> CO + H2) and helps passivate the coil metal surface. It is not the heat source: the heat comes from the furnace burners through the radiant coil walls. Typical ratios are about 0.3 kg steam/kg for ethane and 0.5 for naphtha.

  4. 4.What happens if the temperature in a steam cracker is too low?Application

    Conversion per pass falls, so ethylene yield drops and more unconverted feed must be recycled or leaves in the heavier streams. The product slate also shifts toward propylene and C4s relative to ethylene, i.e. lower severity. Coking is generally lower at reduced temperature, so the trade-off is run length versus yield, and operators set the coil outlet temperature to the economic optimum.

  5. 5.Describe the role of a furnace in the steam cracking process.Concept

    The furnace in the steam cracking process is responsible for providing the necessary heat to achieve the high temperatures required for cracking hydrocarbons. It consists of radiant coils where the hydrocarbon feedstock mixed with steam is heated. The furnace ensures that the feedstock reaches the desired temperature quickly to initiate the cracking reactions, maximizing the production of ethylene and propylene while minimizing byproducts.

  6. 6.How does the choice of feedstock affect the steam cracking process?Application

    The choice of feedstock significantly affects the steam cracking process in terms of product yield and operational conditions. Lighter feedstocks like ethane tend to produce higher yields of ethylene, while heavier feedstocks like naphtha produce a broader range of products, including more propylene and byproducts. The feedstock also influences the temperature and residence time required in the reactor, as well as the formation of coke.

  7. 7.What are the environmental considerations in the steam cracking process?Application

    Environmental considerations in the steam cracking process include the emission of greenhouse gases, such as CO2, due to the combustion of fuels in the furnace. The process also generates byproducts like coke, which need to be managed. Efforts to improve energy efficiency, reduce emissions, and manage waste are crucial to minimizing the environmental impact of steam cracking. Technologies like heat recovery and carbon capture can be employed to address these concerns.

  8. 8.Calculate the amount of ethylene produced if 1000 kg of ethane is fed into a steam cracker with 80% conversion, assuming all converted ethane becomes ethylene.Numerical

    Moles of ethane = 1000 kg / 30.07 kg/kmol = 33.26 kmol. Converted = 0.80 x 33.26 = 26.60 kmol, giving 26.60 kmol ethylene (C2H6 -> C2H4 + H2). Mass = 26.60 x 28.05 = 746 kg of ethylene. In practice molar selectivity is about 80-85%, so the real figure is lower.

  9. 9.What is the impact of residence time on the steam cracking process?Application

    Residence time in the steam cracking process affects the extent of cracking and the product distribution. Shorter residence times generally favor the production of lighter alkenes like ethylene, while longer residence times can lead to over-cracking, producing more byproducts and coke. Optimizing residence time is crucial to maximizing yields of desired products while minimizing undesirable byproducts.

Finished this topic? Mark it so your progress, study plan and readiness keep up.

Stuck on something here?

PreviousSecondary refinery processes: cracking, reforming, hydrotreatingNext Aromatics: benzene, toluene and xylene production